Plant viruses are a serious problem for many of the major agricultural crops, as virus infections cause large harvest losses.
In sugar beet, the major causes of diseases are: (i) yellowing caused by a polerovirus, the Beet mild yellowing virus (BMYV) transmitted by its principal vector Myzus persicae in a persistent manner; (ii) sugar beet rhizomania caused by a benyvirus, the Beet necrotic yellow vein virus (BNYVV), transmitted by Polymyxa betae. Extensive use of resistant against BNYVV permitted to preserve yields, however resistant breaking viral isolates are occurring and there is an urgent need for novel resistant varieties.
Fungus-transmitted viruses, such as BNYVV may be retained in resting spores in soil for years once a field becomes infested. As no effective chemical or physical methods exist for eliminating the virus, neither in the plants nor in the soil, the only option for the sugar beet farmer is the use of genetically resistant cultivars. Several companies have provided a number of tolerant, even partially resistant varieties through breeding. This is, however, a very tedious and time-consuming process, generally taking a long time before useful resistant plants are obtained.
The rapid revolution in the areas of plant engineering has led to the development of new strategies to confer genetic resistance to viruses. Resistance to viral diseases through the introduction of portions of viral genome sequences whereby the viral sequence (construct) is transformed into a plant cell and a plant, has become a new source of resistance.
Sugar beet is known to be recalcitrant species in genetic engineering, complicating a possible successful induction of viral resistance.
A few examples of engineering tolerance, for instance to the BNYVV by transforming and expressing the BNYVV coat-protein sequence in the sugar beet genome, have been published (WO91/13159) though there are only rare report data on whole functional transgenic sugar beet plants, such as those disclosed in EP 1 169 463 B1. In particular, reports show limited data on the level of resistance observed in infected conditions with transgenic sugar beet plants transformed with a gene encoding a BNYVV coat-protein sequence.
The genome of beet necrotic yellow vein furovirus (BNYVV) consists of five plus-sense RNAs, two of which (RNAs 1 and 2) encode functions essential for infection of all plants while the other three (RNAs 3, 4 and 5) are implicated in vector-mediated infection of sugar beet (Beta vulgaris) roots. Cell-to-cell movement of BNYVV is governed by a set of three successive, slightly overlapping viral genes on RNA 2 known as the triple gene block (TGB), which encode, in order, the viral proteins P42, P13 and P15 (gene products are designated by their calculated Mr in kilodalton).
The genome of BMYV consists of a linear plus-sense RNA with six major open reading frames (ORFs 0-5). ORFs 1 and 2 encode proteins involved in virus replication, while each of the other three ORFs (ORFs 3, 4 and 5) codes for structural proteins (major and minor coat proteins) and a putative movement protein.
It has been shown that P0 protein of BMYV has a poor expression, a consequence of unfavorable initiation codon context of the P0 AUG and a strong tendency to maintain a low expression. Furthermore, this part of the genome is highly variable, and this sequence diversity has been exploited to discriminate the different species.
Diseases caused by BNYVV are shown to expand geographically, at a speed depending upon the combination of numerous local environmental and agricultural factors. Therefore there is a need to improve the genetic resistance mechanisms which may, alone or in combination, confer a stable and long lasting resistance of sugar beet plants which are grown for industrial use.